In this application we propose to continue our research into the molecular mechanisms of radiation induced carcinogenesis. We have found that both c-myc and K-ras oncogenes were activated in a panel of large late stage tumors of rat skin induced by a single exposure to ionizing radiation. We have also recently discovered that the activation of c-myc by gene amplification is a late stage event in this model system. The experiments proposed in this application will help to elucidate several important questions in radiation induced cancer, and in carcinogenic mechanisms in general. The major thrust of this proposal is to further clarify the relationship of oncogene activation to the temporal aspects of multistage carcinogenesis in the rat skin model. We will analyze tumor biopsies for activation of myc and ras oncogenes using Southern analysis, PCR and in situ hybridization. These experiments will test the hypothesis that myc activation occurs in a subpopulation of tumor cells, which ultimately enjoys a selective advantage during subsequent progression. The carcinogen and tissue specificities of the oncogene activation patterns we have observed will be determined by comparison with patterns found in tissue block sections of rat skin tumors induced by a chemical carcinogen, and in human skin tumors induced by radiation exposure. Using new applications of the polymerase chain reaction (PCR) methodology for specific sequence amplification, we will determine the earliest time that oncogene activation can be detected in radiation exposed target tissue, before tumor appearance. We have available over 30,000 preserved tissue blocks from 200 experiments dealing with various issues of radiation carcinogenesis. This valuable resource will be tapped to elucidate the relationship of radiation quality with respect to LET to oncogene activation. Radiation induced rat skin tumors will be used for the establishment of cell lines. These lines will be thoroughly characterized including karotype analysis, and will provide useful tools for future research in mechanisms of oncogene activation in radiation carcinogenesis. Increased understanding of the molecular mechanisms associated with skin cancer induced by ionizing radiation will provide useful knowledge for the eventual prevention, management or control of human cancer.